1. Field of the Invention
The present invention relates to an image display apparatus, and more particularly to an image display apparatus having such current-driven light-emitting devices as organic electroluminescence (EL) devices for respective pixels.
2. Description of the Background Art
In these years, in the field of flat panel displays, attention has been being given to organic EL displays in addition to liquid crystal displays. As compared with the liquid crystal display, the organic EL display has a higher contrast ratio, a faster response characteristic and a wider viewing angle. The organic EL display has organic EL devices, which are current-driven light-emitting devices, arranged for respective pixels. As a representative example of the organic EL device, organic light-emitting diode is known.
In particular, in recent years, attention has been being given to low-temperature polysilicon thin film transistor (TFT) displays among those organic EL displays. In consideration of higher image definition and lower power consumption, the low-temperature polysilicon TFT display has thin film transistors using low-temperature polysilicon as drive devices of organic light-emitting diodes. The low-temperature polysilicon TFT display, however, tends to have such transistor characteristics as mobility and threshold voltage that vary to a relatively greater degree depending on manufacture, as compared with conventional TFTs.
With the above-described situations as a background, it has been pointed out that one problem of the organic EL display is nonuniformity in display brightness characteristic between pixels, namely “nonuniform display”. Japanese National Patent Publication No. 2002-517806 for example discloses a configuration of a pixel circuit as a configuration for indicating this problem.
FIG. 7 is a circuit diagram illustrating the conventional pixel circuit disclosed in Japanese National Patent Publication No. 2002-517806.
Referring to FIG. 7, conventional pixel circuit 100 includes, for an organic light-emitting diode OLED provided as a light-emitting device, a pixel drive circuit 110 for supplying electric current according to a specified display brightness.
Pixel drive circuit 110 includes an n-type TFT device Q1 used as a current drive device, a voltage holding capacitor CH and switches S11 to S13. Here, the TFT is described hereinafter as a representative example of field effect transistors.
Organic light-emitting diode OLED is a current-driven light-emitting device and changes in display brightness according to supplied electric current. The anode of organic light-emitting diode OLED is connected to a supply voltage VH.
N-type TFT device Q1 is connected between the cathode of organic light-emitting diode OLED and a supply voltage VL. To supply voltage VL, a ground voltage or a predetermined negative voltage is applied. The gate of n-type TFT device Q1 is connected through voltage holding capacitor CH to supply voltage VL and connected through switch S12 to the drain of n-type TFT device Q1.
Switch S11 is connected between a data line DL and a node N1 at a voltage equal to that of the drain of n-type TFT device Q1.
Switch S13 is connected between the drain of n-type TFT device Q1 and the anode of organic light-emitting diode OLED.
Pixel circuit 100 having the above-described configuration performs its display operation in two modes. In a data write mode corresponding to an addressing cycle, drive current IEL that determines a necessary output from organic light-emitting diode OLED is driven from a constant current source 60 to data line DL.
In pixel circuit 100, switch S11 is turned on to electrically couple data line DL to node N1. Further, switch S12 is turned on to diode-connect n-type TFT device Q1 while switch S13 is turned off to electrically insulate organic light-emitting diode OLED. Accordingly, a current path from constant current source 60 through data line DL and n-type TFT device Q1 to supply voltage VL is formed and drive current IEL is flown through the current path.
FIG. 8 is an equivalent circuit diagram of n-type TFT device Q1 in the data write mode.
Referring to FIG. 8, since n-type TFT device Q1 is in the diode-connected state, n-type TFT device Q1 operates in a saturation region. Further, a gate to source voltage VGS is set to a voltage level necessary for allowing drive current IEL to flow, and held in voltage holding capacitor CH.
Here, drain current (corresponding to IEL) in a saturation region of a field effect transistor like the TFT device is generally represented by expression (1):IEL=(β/2)·(VGS−VTN)2  (1)where β=μ·(W/L)·Cox, and β represents current amplification factor, μ represents mobility, L represents gate channel length, W represents gate channel width, Cox represents gate capacity, and VTN represents threshold voltage.
From expression (1), gate to source voltage VGS is represented by expression (2) as indicated below, with threshold voltage VTN of the transistor to which added an amount of increase in voltage caused by drive current IEL:VGS=VDS=VTN+(2IEL/β)1/2  (2).
Furthermore, switches S11 and S12 are turned off to electrically insulate pixel circuit 100 from data line DL and electrically insulate voltage holding capacitor CH. Accordingly, as a terminal to terminal voltage of voltage holding capacitor CH, gate to source voltage VGS necessary for allowing drive current IEL to flow through n-type TFT device Q1 is stored.
When gate to source voltage VGS is stored in voltage holding capacitor CH and the data write mode is ended, switch S13 is turned on to connect the cathode of organic light-emitting diode OLED to the drain of n-type TFT device Q1 and thereby start a display mode.
In the display mode, in order to generate the output determined by drive current IEL as described above from organic light-emitting diode OLED, n-type TFT device Q1 drives the electric current according to voltage VGS stored in voltage holding capacitor CH to organic light-emitting diode OLED. In other words, n-type TFT device Q1 operates as an electric current source to allow electric current equal to drive current IEL to flow through organic light-emitting diode OLED.
As discussed above, in the data write mode and the display mode, the same n-type TFT device Q1 is used for supplying current and for generating current. Therefore, drive current IEL is kept at a constant level without being influenced by threshold voltage VTN and mobility μ of n-type TFT device Q1.
Generally, a field effect transistor including the TFT device that is used as the current drive device in pixel circuit 100 in FIG. 7 (depending on the case, the field effect transistor is hereinafter referred to as current source transistor) has the relation as shown in FIG. 9 between drain to source current IDS and drain to source voltage VDS.
Referring to FIG. 9, an operation region of the current source transistor is roughly divided into a non-saturation region and a saturation region. In the non-saturation region, drain to source current IDS increases together with drain to source voltage VDS. In the saturation region, a constant current characteristic is exhibited that is determined by only the gate to source voltage VGS regardless of drain to source voltage VDS.
The direct current characteristic represented by the dotted line in FIG. 9 is a characteristic of an ideal transistor having sufficiently large dimensions. In contrast, it is known that an actual fine transistor exhibits a more complicated characteristic, as shown by the solid line, because of the channel length and channel width resultant from the form effect and because of supply voltage.
Regarding the ideal transistor, as shown by the dotted line, once drain to source current IDS saturates, drain to source current IDS remains the same even when drain to source voltage VDS is increased. In contrast, regarding the actual transistor, even in the saturation region, drain to source current IDS slightly increases together with drain to source voltage VDS. Namely, so-called channel modulation occurs, since the effective channel length shortens when an end of a depletion layer of the drain shifts toward the source. In the saturation region, this channel modulation causes a resistance component r between the drain and the source to appear. This resistance component r corresponds to the reciprocal of the channel conductance between the drain and the source.
In pixel circuit 100 in FIG. 7, when switches S11 and S12 are turned on in the data write mode, drain to source voltage VDS according to drive current IEL is set as represented by expression (2). Then, switches S11 and S12 are turned off so that this voltage is held as gate to source voltage VGS in voltage holding capacitor CH.
In the display mode, when switch S13 is turned on, a voltage is supplied through organic light-emitting diode OLED from supply voltage VH to allow current to flow through n-type TFT device Q1. At this time, because of an amount of decrease in forward voltage of organic light-emitting diode OLED (depending on the case, the amount of decrease is hereinafter referred to as VF), a voltage smaller than supply voltage VH substantially by VF, i.e. (VH−VF) is applied to node N1. Accordingly, the voltage on node N1 increases from drain to source voltage VDS of n-type TFT device Q1 to (VH−VF).
Here, as shown in FIG. 9, in the saturation region, actually the channel modulation of n-type TFT device Q1 occurs due to resistance component r, and drain to source current IDS increases as drain to source voltage VDS increases.
If respective n-type TFT devices Q1 of all pixel circuits 100 arranged in rows and columns in a matrix form in a display unit have the same resistance components r, namely channel conductance, an amount of increase in current IDS would be equal between the current source transistors and accordingly these pixel circuits 100 can have uniform electric current driven to respective organic light-emitting diodes OLEDs.
Actually, however, n-type TFT devices Q1 have respective resistance components r different in magnitude from each other due to for example variations depending on manufacture, pixel circuits 100 differ from each other in electric current driven to respective organic light-emitting diodes OLEDs, causing the nonuniform display.